Terahertz (THz) spectroscopy is a promising technique for the stand-off identification of hidden objects. The THz band is particularly well suited firstly because THz radiation penetrates many dielectrics like clothing and secondly because many potentially hazardous substances have characteristic signatures in the THz spectral region. In order to demonstrate the full potential of THz radiation for identifying possible hazards using characteristic signatures, different disturbing influences must be taken into account. We have performed experiments and simulations in order to investigate the possibilities and the challenges of terahertz stand-off identification. A special emphasis is paid on humidity in ambient air and properties of the sample like surface roughness and orientation with respect to the incident THz beam. Water vapor absorption strongly affects the THz spectra. Since the absorption lines are strong and narrow, the calculation must be precise. We have checked models well-known in meteorology covering the infrared and the microwave regions of the electromagnetic spectrum and achieved an accurate description of the measured THz spectral absorption using the program LINEFIT. The surface roughness of the sample strongly affects the bandwidth of the reflected spectra. Specular and diffuse reflection measurements using samples with different roughnesses have been used for determining the influences of different properties on the reflection spectra.